1. Field of the Invention
This invention relates generally to an apparatus and method for controlling crankshaft idle speed during a vehicle creep condition in a hybrid electric vehicle (HEV).
2. Description of the Prior Art
A powershift transmission is a geared mechanism employing two input clutches used to produce multiple gear ratios in forward drive and reverse drive. It transmits power continuously using synchronized clutch-to-clutch shifts.
The transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear.
During a vehicle creep condition while the engine is idling in a conventional vehicle having an engine, powershift transmission, single propulsion path and single power source, the transmission clutch torque capacity is controlled by slipping the transmission input clutch as the driver releases the brake pedal. In a powershift transmission vehicle application, providing consistent, acceptable vehicle creep performance can be a difficult control problem due to the absence of a torque converter.
As the driver releases the brake pedal, the increase in clutch torque capacity loads the engine and disturbs the control of the engine idle speed. Therefore, engine idle speed control must be coordinated with any increase in the clutch torque capacity.
Unlike a conventional vehicle having a powershift transmission, a hybrid electric vehicle with a powershift transmission, multiple power sources can be used during a vehicle creep condition to provide robust, responsive engine idle speed control while accounting for the battery charging needs of the vehicle.
A need exists for responsive idle speed control that corrects for input clutch torque capacity disturbance, delayed engine torque response due to intake manifold filling as clutch torque capacity is increased, and potential engine stall if too much clutch torque capacity is provided while the engine torque has not increased sufficiently. The engine idle speed control must provide good coordination between transmission clutch torque capacity control and crankshaft speed control during a vehicle creep condition.